This invention relates to an electromechanical transducer for implantable hearing aids for average to severe hearing damage, whose cause is located basically in the inner ear.
Since a hearing impairment is still regarded as a "handicap," today the trend in the development of conventional hearing aids has for many years been up to the limit of the feasible miniaturization of the devices. With regard to this aspect, the "In-the-Ear Devices" (ITE) (e.g., German Offenlegungsschrift 28 25 233) have made good progress and are today more widely used than "Behind-the-Ear Devices" (BTE) (e.g., European Patent 0 341 902). Miniaturization, however, has the following drawbacks:
distinctly lower acoustic amplification in comparison to BTE devices; PA1 inadequate sound quality, which is attributable basically to nonlinear distortions at high output sound pressure levels and poor frequency response of the "loudspeakers" (hearing aid receivers); this inferior quality is basically caused by the transducer principle (electromagnetic) and the extremely small design. Both factors hardly allow for more improvements, since here the physical limits have already been more or less reached; PA1 acoustic feedback "howling," which occurs when the device does not fit tightly into the outer ear canal and with high acoustic amplification; PA1 despite being placed in the outer ear canal, most devices remain visible; this important aspect is a psychological problem (stigmatization) known for a long time and plays a significant role, e.g., during puberty or at important social or business functions; PA1 the tight fit of the ITE device, necessary for acoustical reasons, in the outer ear canal causes the unpleasant "locking" effect ("foreign body" in the outer ear canal). PA1 In almost all publications and patents, the use of piezoelectrically active transducer elements, which are produced only as a bimorphic structure or in a multilayer technique, is proposed. The principle of the bimorph means that two strips of piezoelectrically active material are joined together mechanically so that in simultaneous electrical stimulation of the two elements mechanically permanently clamped on one side, one is shortened because of the piezoelectric transverse effect and the other is correspondingly lengthened, by which a distinctly greater deflection and thus deviation of the free end of this connecting element is achieved than with only one transducer strip. In joining together more than two of these strips (multilayer technique), the producible force can additionally be increased. Largely, optimal use is then made of this principle if the geometry of the connecting elements is designed as rectangular strips, and the achievable deviation with the given transducer voltage is increased with increasing length and decreasing thickness of the connecting elements. PA1 Although other physical transducer principles are mentioned (in particular electromagnetic), it is explained that technically simple transducer elements are achievable only with the piezoelectric effect. PA1 The sound quality achievable with the stimulation of the ossicle chain and in particular of the stirrup directly with the above-mentioned bimorphic elements is apparently very high. PA1 In some patents and publications, references to the necessary biocompatible sheathing and high-grade electrical insulation of these piezoelectric transducer elements are indeed found; detailed embodiments of such transducer designs are, however, if mentioned at all, are only generally indicated. PA1 The experiences of many years with implantable, active human implants, such as pacemakers and cochlea implants, show that such systems have to be hermetically sealed, to meet the requirement of clinical safety and a long service life. The concept of hermetic sealing is mentioned, however, in none of the above-mentioned patents; in particular, no implantable hearing aid with a transducer design is described as being hermetically sealed. PA1 Nor is it described how such a transducer design has to be made, so that the transducer also is actually implantable, i.e., is configured so that the operating surgeon retains a substantially free view of the operating area (in particular of the ossicle chain and the oval and round window); further, the transducer has to be designed so that its application in the middle ear area is possible under the given anatomical conditions, and no substantial surgical curettage has to be performed, which is expensive and entails the high risk of traumatic reactions and subsequent infections. This aspect contradicts the use of piezoelectrical bimorphic transducer elements, since the latter--as mentioned--have to be technically optimally extended and require a permanent clamping of one end (minimum useful length about 7 mm; this measurement already exceeds anatomically average geometries in the middle ear). Such design features are also not indicated in any patent. PA1 Direct transducer material data for piezoelectric elements relates only to concepts such as "piezoelectrically active ceramic" or PVDF (polyvinylidene fluoride).
A fully implantable hearing aid with an optimized electromechanical transducer for direct mechanical stimulation of the middle or inner ear, which does not exhibit the above-mentioned drawbacks and moreover has substantial improvements, is suitable in hearing impairments whose cause is located in the inner ear or in higher auditory processing levels and cannot be corrected by surgical measures. Worldwide, the share of such inner ear impairments (offending noises, impairment caused by disease, ototoxic medications, presbycusis, etc.) clearly exceeds (about 80%) the cases of a middle ear defect (disturbance of sound conduction) which can be corrected by surgery.
For said reasons, tests have been made for quite some time to use principles of (inner) ear stimulation other than the reconversion into amplified airborne sound. Here, e.g., the possibilities of partially implantable, electromagnetic systems can be mentioned, in which one of the auditory ossicles in the middle ear (malleus, incus and stapes) is permanently joined mechanically with a very small permanent magnet; a (conventional) ITE hearing device now does not control a hearing aid receiver, but a magnetic induction coil whose alternating field, which corresponds to the acoustical data, stimulates the implanted magnet and thus the ossicle chain to oscillations, which are greater in their amplitude with a suitable system design than in the natural case and thus cause the amplifying effect. In this case, the actuating coil is brought, from outside, as close as possible to the tympanic membrane, to keep the air gap to the implanted magnet to a minimum. Other embodiments consist in the replacement of one of the ossicles (e.g., anvil) with one made from a biocompatible material which sheathes a permanent magnet. Such partially implantable electromagnetic systems are described in the literature, e.g., in Heide et al. (Adv. Audiol., vol. 4, pp. 32-43, Karger, Basel 1988) and documented in numerous patents (U.S. Pat. Nos. 3,870,832; 4,606,329; 4,756,312; 5,015,224; and 5,015,225, German Patent No. 3 617 118 C2; European Patent No. 0 242 038; UK Patent Nos. 1 440 724; and 2 188 209.
In addition, systems are also known which stimulate the inner ear by a body-borne sound (bone) conduction (U.K. Patent No. 2,176,078, partially implantable electromagnetic; and U.S. Pat. No. 4,150,262, nonimplantable piezoelectric).
Most of the above-mentioned processes and devices contain partially implantable transducers to stimulate the middle ear ossicles or, directly, the oval window.
A possible principle of a fully implantable electromechanical transducer for direct mechanical stimulation of the stirrup is described in publications of the Japanese group of Yanigahara and Suzuki et al. (Arch Otolaryngol Head Neck, Surg-Vol 113, 1987, pp. 869-872; Hoke, M. (ed), Advances in Audiology, Vol. 4, Karger Basel, 1988).
In this system, the electromechanically active part consists of a piezoelectric ceramic bending transducer which is designed as a bimorph and directly actuates the head of the stirrup with its free oscillating end by a small coupling element (e.g., made from polyethylene). The mechanically necessary permanent clamping of the other end of this bending transducer is achieved by complicated adjusting rods made from titanium, whose fastening element is bolted to the bone. Tests on humans under local anesthesia show that deviations of the stirrup are achievable with this transducer element in the case of transducer voltages around 1.0 volt, which correspond to equivalent stimulating sound levels of 90-100 dB SPL in the audiologically important frequency range of about 250-4000 Hz and thus really are sufficient for a hearing aid supply. An important additional result of these tests is the confirmed high sound quality and freedom from distortion of this type of stimulation of all test subjects, which is also documented in the result quotas of speech comprehension tests in test subjects with distinct inner ear hearing impairments.
In the above-mentioned publications, it is clearly pointed out that a basic problem in the technical achievement of the described transducer elements is the biocompatible sheathing with long-term stability of the freely oscillating piezoelectric ceramic element, which, moreover, also has to have high electrical insulation values in the range of several MOhm since such ceramic elements have a high electrical impedance because of their low quiescent capacity. A further difficulty consists in the space requirement of such transducer elements and the related positioning mechanisms, which obstruct a safe and reliable application in the middle ear area, taking into consideration the individual fluctuations of the anatomical geometries. Further, an anatomically normal and thus functioning ossicle chain has to be broken to be able to use the above-mentioned transducer element; when the loss or other technical or clinical problems occur, which indicate the removal of the transducer, therefore, a surgical reconstruction of the chain has to be attempted to restore the former state.
A similar process is indicated by Epley (U.S. Pat. No. 3,712,962), who also uses piezoelectric bending transducers with coupling to the stirrup as a transducer, which are designed as a bimorph or use a multilayer technique. Here, the necessity of a suitable sheathing is also pointed out (flexible plastics, silicone); also, an embodiment of coupling elements of the free bending oscillator end to the stirrup is described.
Nunley et al. (U.S. Pat. No. 3,882,285) describe a process, in which, parallel to the anatomically normal and intact ossicle chain, the stirrup or the oval window are directly oscillated by a piezoelectric element to "support" and to amplify the natural method of transmission. A detailed embodiment of the transducer is not indicated. In this connection, it is to be noted that interferences can occur in the case of the nonideal phase frequency characteristic of the deviation of the transducer in comparison to natural transmission, which entail considerable breaks in the overall frequency range and thus can very negatively influence the transmission quality and the amplification.
Similar data is documented in Wingrove (U.S. Pat. No. 3,594,514), who also uses a piezoelectric bimorphic element to stimulate the stirrup or oval window, which is to be embedded on one side permanently in the mastoid bone and exhibits a suitable biocompatible encapsulation. Detailed embodiments or descriptions are not indicated there either.
Additional, varied types of stimulations of the stirrup or oval window with piezoelectric bending transducers in a bimorphic type of construction are also found in Branch et al. (U.S. Pat. No. 3,764,748); detailed embodiments of the possibilities of how such transducer elements can be biocompatibly sheathed and electrically insulated, are not mentioned either.
The most detailed embodiments of suitable transducer processes for fully implantable hearing aids are found in Schaefer (U.S. Pat. No. 4,729,366, European Patent No. A1 0 263 254). Here, a method of hearing improvement and a fully implantable device are described, in which two electromechanical transducers, which also preferably are designed as piezoelectric bimorphic elements, are inserted in the broken chain as an "active link" after an interruption of the ossicle chain (typically by removing the anvil as a link between the hammer and stirrup). One of the transducers performs a microphone function by mechanical coupling to the tympanic membrane or the hammer, and the thus obtained electrical signal is fed by an electronic system amplified to the output transducer. This stimulation transducer in turn also stimulates the stirrup or the oval window.
In the above-mentioned patents of Schaefer, conceivable embodiments of these transducers are described to the effect that, preferably, piezoelectric bending transducers are clamped on one side in housings, not described in more detail, and the coupling of the free, oscillatory end of these transducers to the ossicles, preferably, takes place by a thin wire of stainless steel, and in the output transducer this wire is looped, e.g., around the head of the stirrup. The lateral guiding of this wire by the wall of the housing, which contains the active transducer element, takes place by a guide bushing which is embedded in the housing wall. However, a precise explanation of this embodiment is not indicated. The active transducer elements can be piezoelectric (piezoactive polymers such as PVDF or piezoelectric ceramics, preferably with a bimorphic structure) or electromagnetic.
The above-indicated prior art of fully implantable electromechanical transducers for hearing aids can be summarized as follows:
Attempts at solving full encapsulation of implantable hearing aid transducers are indicated only in two of said patents (U.S. Pat. No. 4,988,333 and German Patent No. 3 918 086). Here, active transducer elements in closed housings are used, whose electrically stimulated mechanical oscillations are decoupled by hose lines; the oscillations are further conveyed in these hose lines in liquid or gaseous media and guided either to the head of the stirrup or directly by a liquid coupling through the oval or the round window into the perilymph of the cochlear basal turn. A significant advantage of this special hydromechanical coupling (German Patent No. 3 918 086) consists in that sizable deviations in comparison to the driving transducer membrane, and thus high equivalent output sound pressure levels, are achievable on the basis of the hydraulic principle at the end of the coupling element. Questions of germ-free filling with liquid and of the long-term stable coupling to the middle or inner ear are problematical here with respect to the technical implementation.